Help with a multi scaled continuity tester

Discussion in 'The Projects Forum' started by tom7891, Feb 14, 2014.

  1. tom7891

    Thread Starter New Member

    May 16, 2011
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    Hey, I'm building a continuity tester as a project which will detect different levels of resistance.
    For example, an LED will light up when less than 10 ohms is detected, and another will light up when less than 100 ohms is detected.
    [​IMG]

    Each scale (less than 10 and less than 100) will be built from a comparator, which will compare a set calculated voltage (say, 0.9v) against the voltage produced with the external measured resistance connected (using ohms law).
    I've got each comparator to work with separate switches, the problem arises when i attempt to connect the two op amp circuits to a single switch that will simulate the two probes (Bottom left of the circuit).
    I see why it is happening, i'm basically connecting the two circuits together which interferes with the resistances, but i was wondering if anyone on here knew a way around it? I dont think there is, but there may be a method that i'm not aware of.
    Thanks for any help, and sorry if i'm not explaining it properly, i'm a beginner at electronics to be honest.
     
  2. inwo

    Well-Known Member

    Nov 7, 2013
    2,441
    315
    Haven't had a chance to look at circuit.

    The title got my attention.

    I'm sorry if this seems a hijack. I'm interested in your project and will be watching it progress.:)

    I once made a very similar device for production.

    Never caught on. Looked all over for schematics without luck. :)

    Anyway it used just two major components.

    An LM3915 http://www.ti.com/lit/ds/symlink/lm3915.pdf or similar.

    This has all the comparators built in.

    And a matching 10 segment bar graph display.
    Actually, the production one may have used discrete leds and labels on the face.

    Remember using a constant current source of I believe 1ma.

    Packaged with 10 resistors and clip leads it made a very versatile tester.

    Ohms test, constant current source, 10 lead tracer.

    The prototype was stolen while I was working at a school and never got any further.:(
     
    Last edited: Feb 14, 2014
  3. crutschow

    Expert

    Mar 14, 2008
    16,508
    4,441
    Don't quite understand your circuit. What is the purpose of the transistors? They and LEDs D2 and D5 should not be in the measurement part of the circuit as they reduce the measurement accuracy.

    To avoid interaction between the two circuits use a single pair of resistors to provide voltage to the resistance to be measured and connect the junction of those resistors to one input on each comparator. They you can use different ratio resistors on the comparator's other inputs to get your two independent trip levels.

    You don't necessarily need a constant current to the resistance under test. Your bridge approach should work fine.
     
  4. tom7891

    Thread Starter New Member

    May 16, 2011
    26
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    Thanks for the reply. The transistors I think are now redundant. They came from an earlier idea and they've just remained there haha.
    Yeah the two LEDs are there to provide a constant current to base the input voltage off of. I'll try getting rid of those later if you're saying that I don't need them :)
     
  5. crutschow

    Expert

    Mar 14, 2008
    16,508
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    The LEDs will adversely affect the circuit accuracy. You don't need a constant current, just the 4 resistors to provide a bridge circuit (actually 6 resistors for the two comparators) since a bridge circuit accuracy is largely unaffected by the supply voltage.
     
  6. tom7891

    Thread Starter New Member

    May 16, 2011
    26
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    Thanks for the help :) I've got it sorted now and it's pretty accurate (within 0.2 of an ohm). I used the testing resistance as the second bridging resistor. Here's the circuit if anyone's interested:
    [​IMG]
     
  7. crutschow

    Expert

    Mar 14, 2008
    16,508
    4,441
    You can save considerable battery current by making the resistors a larger value, since those low values will draw over 0.8A from your battery, far more than a 9V can provide (at least for only a short time). The input requires 0.45A but that's only for the time you probe the circuit.

    If the resistors have the have the same ratio, the circuit will operate properly. Thus instead of 1Ω and 20Ω for R10 and R9, you can use 1kΩ and 20kΩ, for example.

    As inwo noted, the easiest way to do this circuit is with an LM3915 dot/bar generator which has built in comparators and constant-current LED drivers for up to 10 LEDs. It has a log response which matches the large range of resistor values you want to detect. That would greatly simplify your circuit to one IC and a few resistors.
     
  8. tom7891

    Thread Starter New Member

    May 16, 2011
    26
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    Yeah i agree about the resistors, good idea :)
    I agree that using that ic would be easier and i would use it, but this is a college project based around op amps so it's probably best to use a quad op amp chip :)
     
  9. inwo

    Well-Known Member

    Nov 7, 2013
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    Sorry, as it was in projects forum, I had to suggest.:D
     
  10. tom7891

    Thread Starter New Member

    May 16, 2011
    26
    0
    Yeah cheers for the suggestion :) It's good to know anyway, I'm still learning so it's good to know these things.
     
  11. tom7891

    Thread Starter New Member

    May 16, 2011
    26
    0
    Hi again, I just want to ask another question. I've built the circuit up now but I'm not getting the results I intended.
    For some reason the voltage comparator (lm324n) isn't comparing voltages correctly, for example, on one instance I've got 6.2v on the positive terminal and 6.5v on the negative, yet I still get an output. And then I get 5.9v on the positive with 6.5v on the negative yet I still get an output. Any ideas? I don't know how accurate these comparators are, is there enough of a voltage difference? Thanks.
     
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